Domestic dwellings are frequently fitted with balanced flue condensing gas boiler systems. In such an arrangement it is common for the boiler to be fitted on the interior face of an external wall of the building and a relatively short flue to be passed through the wall so as to vent outside. As a result, the flue tends to vent quite near to the ground, and often just at or slightly above head height.
A condensing boiler deliberately seeks to keep its flue gas temperature as low as reasonably possible. The flue gas naturally has to be warmer than the maximum water temperature required of the boiler, but any unnecessary increase above that results in heat being lost to the atmosphere and hence the efficiency of the boiler being reduced. Typically a condensing a boiler providing domestic heating can operate in its condensing mode and maintain a flue gas temperature below 58° C. However when operating in a hot water mode the boiler must seek to maintain its target temperature rise and flow rate and this causes the boiler to work in a less efficient regime where the flue gas temperature exceeds 58° C. and may reach 75° C.
A disadvantage of this move to lower flue gas temperatures is that the moisture within the flue gas formed as a by-product of the combustion process is far more likely to condense in the vicinity of the flue outlet. This can often be observed on windless days as the formation of a white cloud or plume around the flue vent. The formation of this plume is visually unattractive, especially in high density housing where several individual heating systems may vent in close proximity within, for example, a shared courtyard environment. Furthermore, as the moisture tends to concentrate other combustion products, such as acids, the formation of such low level plumes is undesirable from a public health stand point.